Usefulness of thoracic radiotherapy is greatly limited by lung sensitivity to irradiation doses necessary to eradicate malignant cells. Clinically significant radiation toxicity occurs in up to 30% of patients irradiated for lung cancer and about 10-15% of other thoracic oncology patients. The need to protect normal lung parenchyma from radiation injury compromises delivery of tumoricidal radiotherapy doses. Therefore, radioprotectors that would increase the therapeutic ratio of radiotherapy are urgently needed. Reactive oxygen species induced by ionizing radiation contribute significantly to the pathogenesis of fibrotic lung disease. However the molecular pathways from the oxidative tissue insult to late radiation fibrosis are unclear and at this time, no free radical scavenger offers acceptable levels of pulmonary radioprotection without serious side effects. We and others have shown that antioxidant enzyme therapy alleviates radiation- induced fibrotic lung disease and our preliminary data show that wholegrain flaxseed (FS) or FS lignans, bioactive compounds with known antioxidant, anti-inflammatory and anticarcinogenic properties, induce transcriptional activation of antioxidant enzyme genes in mouse lungs. Furthermore, when evaluated in our mouse model of thoracic radiation-induced pneumonopathy, whole grain FS diet robustly increased mouse survival, abrogated lung fibrosis and decreased oxidative tissue damage while radiosensitizing lung tumor. Therefore, we hypothesize that dietary FS lignan-mediated activation of antioxidant genes may be a novel therapeutic strategy to alleviate radiation pneumonopathy. In the current study, in Aim 1 we will explore the potential pulmonary radioprotective efficacy of FS lignans in a well-established murine model of radiation lung injury using dietary formulations of Secoisolariciresinol Diglucoside (SDG), the main FS lignan precursor. In Aim 2 we will determine if ARE activation is implicated in the mechanism of protection and establish their cell specificity of action. In Aim 3 we will evaluate lignan-mediated radiosensitization of lung cancer cells using a novel K-ras mouse model of orthotopic lung cancer. Successful completion of the proposed studies will elucidate the mechanism and determine the usefulness of FS lignan-mediated radioprotection. SDG is currently being evaluated in clinical trials worldwide a fact that will facilitate ultimately testing the findings of this proposal in the clinic on lung cancer radiotherapy patients.